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If you hear the word “baby-faced”, you might picture a doe-eyed celebrity who never seems to age. But, really, you should be thinking of the axolotl.

These Mexican salamanders are a textbook example of neoteny, which is a critter’s ability to retain juvenile features into adulthood. Basically, axolotls look like salamander larvae even after they become sexually mature. And while many species, including humans, demonstrate neotenous traits, axolotls are extra weird because, with the right chemistry, scientists can force them to basically grow up in appearance.

By messing with hormones, it’s possible to transform these otherwise forever-babies, causing them to lose their youthful glow and look like any other salamander. Generally, amphibians experience metamorphosis, in which their bodies undergo dramatic changes as they develop into adults. Think of a tadpole, which reabsorbs its tail and grows legs, morphing from a fully-aquatic baby into a terrestrial toad!

During adolescence, other species, like the tiger salamander, can look a lot like axolotls. They have frilly external gills for breathing underwater and fins on their tails for swimming around. During metamorphosis, they lose these aquatic features in preparation for life on land.

But axolotls are the Peter Pans of the amphibian world. Instead of metamorphosis, they undergo paedomorphosis, retaining a number of juvenile traits while still developing into an adult capable of reproducing. Some scientists think that this refusal to grow up was an evolutionary response that enabled them to exploit aquatic environments that didn’t dry up seasonally.

If the water is worth sticking around in, they might as well just never leave. But it turns out it is possible for an axolotl to metamorphosize! The key: thyroid hormones.

These hormones are actually vital for metamorphosis in all amphibians, and we’ve known that for some time. In the early 1900s, scientists were trying to sort out the different influences organs had on growth and development, and to learn more, they would actually feed those organs to laboratory animals. And when you feed another animal’s thyroid to an amphibian, it jumpstarts the metamorphosis process, suggesting there must be something in there that affects their development.

In 1920, a researcher first tried this out on axolotls, and found they matured into full-blown adulthood. Fortunately, we’ve since identified the hormones involved, and can just add those to the water. The main thyroid hormones at play are triiodothyronine, or T3, and thyroxine, called T4.

They regulate genes that drive the unique physical changes during this period of development. Compared to their terrestrial relatives, axolotls have much lower thyroid hormone levels, both as juveniles and adults. And they seem particularly low on T4 in their plasma.

But it takes multiple genes to control something as complex as hormone signaling. So even though axolotls don’t have much of the hormones, they’ve retained a lot of other necessary pieces, because they evolved from an ancestor that did fully morph. Which is why scientists can, and have, induced full metamorphosis in axolotls by upping their thyroid hormone intake!

And it’s not just external features like their gills and tail that change. Even their lungs and heart transform dramatically, essentially becoming a functional terrestrial adult. This might seem like a strange experiment, but it helps researchers study what’s really happening during paedomorphosis and metamorphosis, both at the cellular level and from an evolutionary perspective!

Such a weird test case could shed some light on the influence of thyroid hormones like T4 in vertebrate brain development. It could also help us understand how some animals regrow limbs. Axolotls do this easily in their forever-baby state, but lose the ability when made to grow up.

Some scientists think that could be related to changes in their bacterial communities during metamorphosis. So despite their childish lifestyles, we still have a lot to learn from axolotls! Thanks for watching this episode of SciShow, and thanks to our patrons, who are not only generous but also cuter than a whole aquarium full of axolotls.

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